1. Field of the Invention
The present invention relates to a dipole ring magnetic field generator.
2. Description of the Related Art
A typical dipole ring magnetic field generator comprises a plurality of permanent magnet pieces arranged in such a ring that the magnetization direction of each permanent magnet piece makes one rotation over a half circumference of the ring so as to generate a substantially unidirectional magnetic field in the internal space, wherein each permanent magnetic piece has a magnetic field of the same intensity as that of the unidirectional magnetic field. Such a dipole ring magnetic field generator is used in an extensive range of applications, including a magnetic resonance imager (MRI), a semiconductor device manufacturing process, and a uniform magnetic field generator for basic researches (e.g. JP 2005-56903A). Conventionally, normal conductive electromagnets, superconductive electromagnets and the like have been used as the means for generating a uniaxial uniform magnetic field. However, the recent development of high-performance rare earth permanent magnets is causing the rare earth permanent magnets (hereinafter may be referred to simply as “the permanent magnets” or “magnets”) to be used dominantly as a uniform magnetic field generator in the application of a low magnetic field of, for example, IT or less.
A conventional dipole ring magnetic field generator, the permanent magnet pieces comprised by the generator and others will be explained with reference to FIG. 4. FIG. 4 exhibits a schematic sectional view in a plane perpendicular to the central axis of a conventional dipole ring magnetic field generator 6. The dipole ring magnetic field generator 6 comprises a plurality of permanent magnet pieces 701 to 724 and an outer rim yoke 8 surrounding the permanent magnet pieces. As the permanent magnet pieces 701 to 724 in FIG. 4, for example, Nd—Fe—B base, Sm—Co base, or Sm—N—Fe base rare earth permanent magnets, each having an approximately trapezoidal shape or fan-like shape, are used. Further, as the outer rim yoke 8, an annular ferromagnetic or nonmagnetic material is used. The division number of the magnets comprised by the magnetic field generator, i.e. the number of the permanent magnet pieces, may range from about four to about sixty. The number of the magnet pieces is typically selected to be in the range of about twelve to about thirty-six in consideration of high magnetic efficiency and easy circuit fabrication. FIG. 4 exhibits an embodiment of twenty-four divisions.
The permanent magnet pieces 701 to 724 are individually magnetized at a specific cycle with respect to a radial direction. The permanent magnet pieces exactly opposing each other from the central axis of the ring comprised by the magnetic field generator 6 are magnetized with an angle difference of 360 degrees from each other. Adjacent permanent magnets are typically magnetized with angle differences indicated by equations (1) and (2).
                                                        θn              =                                                -                                      360                    N                                                  *                n                                                                        (                                                n                  =                  1                                ,                2                ,                …                ⁢                                                                  ,                                  N                  /                  2                                            )                                                          (        1        )                                                                    θn              =                              360                ⁢                                  (                                                            n                      N                                        -                    1                                    )                                                                                        (                                                n                  =                                                            N                      /                      2                                        +                    1                                                  ,                                                      N                    /                    2                                    +                  2                                ,                …                ⁢                                                                  ,                N                            )                                                          (        2        )            
wherein “θn” represents magnetization direction of n-th magnet, “N” represents division number (natural number) of magnetic circuit, and “n” represents segment number (natural number).
A substantially unidirectional magnetic field, such as the magnetic field having the direction of main magnetic field component (A) in FIG. 4, is generated in the internal space of the ring of the dipole ring magnetic field generator 6 as a result of the magnets configuration described above. If the direction of main magnetic field component (A), which is an X-axis direction in FIG. 4 and which is generated in the internal space of the ring of the dipole ring magnetic field generator 6, is defined as zero degrees, then the angle of a magnetic field vector at any given point of the internal space (hereinafter may be referred to as “skew angle”) is almost zero degrees at the center of the internal space, while the angle of the magnetic field vector tends to deteriorate, i.e. increase, toward the inner wall of the magnetic field generator, due to the characteristics of the magnetic field generator.
When a typical dipole ring magnetic field generator is used, a magnetic field component having a large skew angle is frequently regarded as an impurity, i.e. a noise. Especially a skew angle component (B) on the plane perpendicular to the central axis of the magnetic field generator in FIG. 4 is considered to significantly affect the performance of an element to be produced in the manufacturing process of, for example, semiconductor substrates or the like. For this reason, the skew angle component (B) is required to be controlled to a minimum.